There is an abundant supply of lower alkanes (i.e., (C.sub.1 -C.sub.4)alkanes) and relatively few means of converting them to more valuable products. Although natural gas is predominantly CH.sub.4, it can also contain up to about 15 mole% C.sub.2 H.sub.6, C.sub.3 H.sub.8, and C.sub.4 H.sub.10. Natural gas is currently underutilized primarily due to the difficulties associated with transportation from remote sources. Therefore, much research has been devoted to investigating the conversion of methane to more easily transportable products, such as synthesis gas (i.e., "syngas" or a mixture of CO and H.sub.2), via catalytic partial oxidation, which can be represented by the following reaction scheme: CH.sub.4 +1/2O.sub.2 .fwdarw.CO +H.sub.2. See, for example, Hickman et at., J. Catal., 138, 267-282 (1992), and Hickman et al., Science, 259, 343-346 (1993).
Catalytic partial oxidation of hydrocarbons results in gaseous products containing varying amounts of carbon monoxide, carbon dioxide, olefins, and oxygenates (i.e., oxygen-containing compounds) such as formaldehyde, acetaldehyde, acetone, etc. Catalytic partial oxidation processes offer great potential for fast, efficient, and economical conversion of lower alkanes associated with remote sources of natural gas into more valuable liquid fuels and chemicals, such as oxygenates. For example, the partial oxidation of butane is an exothermic reaction and can result in a variety of products, such as formaldehyde, acetaldehyde, and butene. These "intermediates," however, are easily converted to CO, CO.sub.2, lower (C.sub.2 -C.sub.3) olefins, and H.sub.2 O, for example, at high temperatures. Thus, the selective formation of the intermediates, particularly the oxygenates, which are generally more valuable, is difficult to accomplish.
It is particularly desirable to form these more valuable intermediates on a commercial scale using partial oxidation because of the resources it could save. For example, formaldehyde is typically produced by a multistep process, which involves conversion of methane to syngas, the syngas to methanol, and finally the methanol to formaldehyde. This multistep process is relatively expensive. Thus, it would be desirable to develop a less expensive and direct route to the formation of oxygenates such as formaldehyde, for example.
To accomplish this, an important goal of catalytic partial oxidation processes is attaining high surface reaction rates, which decreases the contributions from nonselective homogeneous reactions that can result in complete oxidation and decomposition of oxygenates, for example, and thus allow selective catalytic partial oxidation processes to dominate. Sufficiently high surface reaction rates, with few nonselective homogeneous reactions occurring, are very difficult to achieve.
One means by which high surface reaction rates are achieved is by heating the reactants prior to contacting the catalyst, particularly if the catalyst has a low activity. However, preheating the reactants, such as by conventional heat transport through the walls of a reactor, can produce significant thermal reaction before the reactants contact the catalyst and form the complete oxidation products, not the desired intermediates.
Systems containing porous .alpha.-alumina monoliths (e.g., blocks of ceramic foam) coated with high activity catalysts, such as Rh (for syngas) or Pt (for olefins), or metal or metal-coated gauze (e.g., wire screens) 1-10 layers thick, can rapidly heat reactants to the reaction temperature and thereby provide essentially complete conversion of the reactants (i.e., typically CH.sub.4 and O.sub.2). Residence times of only about 10.sup.-3 second are required in such systems. See, for example, Hickman et al., Catal. Lett., 17, 223-237 (1993); Hickman et at., Science, 259, 343-346 (1993); Hickman et al., AIChE, 39, 1164-1177 (1993); and Huffet at., J. Phys. Chem., 97, 11815-11821 (1993).
In these systems, the gaseous reactants (i.e., the feed gas stream or feed gas mixture) remain at nearly room temperature until they enter (i.e., contact) the catalyst, at which they are heated rapidly to about 1000.degree. C. by the exothermic oxidation reactions occurring at the catalyst. In such systems, because reaction heat is generated directly on the surface of the catalyst, the feed gas stream temperature rises from about 20.degree. C. to about 1000.degree. C. in less than about 10.sup.-4 second. In contrast, if the feed gas stream is heated by heat transfer through the reactor wall, such a temperature rise would take about 0.1 second.
Although these high mass transfer rates decrease the contributions from nonselective homogeneous reactions that can result in complete oxidation and decomposition of oxygenates, few oxygenates are actually produced using these systems. The major products are typically olefins having fewer carbons than the starting material (i.e., cracked olefins), CO, CO.sub.2, and H.sub.2 O (the relative amounts depend on the prevalance of the water gas shift reaction).